1. Field of the Invention
The present invention relates to a thin metallic sheet for a shadow mask having high etching performance and particularly to a shadow mask thin metallic sheet made of Fe-Ni alloy suitable for a color cathode ray tube.
2. Description of the Related Art
Recent up-grading trend of color television toward high definition TV has employed Fe-Ni Invar alloy containing 34-38 wt. % of Ni as the alloy for the shadow mask to suppress color-phase shift. INVAR alloy is a low-expansion alloy containing 36% nickel, 0.35% manganese and the balance iron with carbon. The Fe-Ni Invar alloy which contains 34-38 wt. % of Ni is hereinafter referred to as "conventional Fe-Ni alloy". Compared with low carbon steel which has long been used as a shadow mask material, the conventional Fe-Ni alloy has considerably lower thermal expansion coefficient. Accordingly, a shadow mask made of conventional Fe-Ni alloy raises no problem on color-phase shift coming from the thermal expansion of shadow mask even when an electron beam heats the shadow mask.
Common practice of making a shadow mask from a thin alloy sheet includes the following steps. The alloy sheet is photo-etched to form the passage-holes for the electron beam on the thin alloy sheet for shadow mask. The passage-hole for electron beam is hereinafter referred to as "hole". The thin alloy sheet for shadow mask perforated by etching is hereinafter referred to as "flat mask". (2) The flat mask is subjected to annealing. (3) The annealed flat mask is pressed into a curved shape of cathode ray tube. (4) The pressformed flat mask is assembled to a shadow mask which is then subjected to blackening treatment. However, the Invar alloy of conventional Fe-Ni is inferior to the shadow mask material of low carbon steel in terms of etching performance to prepare many micropores.
Conventional Fe-Ni INVAR alloy is considerably weak in corrosion resistance to etching liquid and has large crystal grain size. Compared with mild steel. The result is that light penetrating through the micropores formed by the etching process results in a blurred periphery of the pierced holes of the flat mask. Also, the brightness of light penetrated through the flat mask of conventional Fe-Ni Invar alloy is inferior to that of mild steel. Such a degraded brightness of flat mask is a serious disadvantage in the recently emphasized demand for bright screens. To cope with the problem on etching performance, the prior art 1 and the prior art 2 have been presented.
The prior art 1 is introduced in JP-B-H2-9655 (the term "JP-B-" referred to herein signifies "examined Japanese patent publication"). The patent describes that precise and uniform etching is performed by aggregating {100} plane by 35% or more onto the surface of thin Invar alloy sheet. The flat mask prepared by the method, however, still has hazy photo-irregularity and weak brightness of flat mask, which are left as quality issues.
The prior art 2 is described in JP-A-S62-2437825 (the term "JP-A-" referred to herein signifies "unexamined Japanese patent publication"). In the patent, an aggregated {100} plane onto the rolled plane of Fe-Ni Invar alloy gives the surface roughness Ra in a range of 0.2 to 07 .mu.m and Sm at 100 .mu.m or below, and gives the crystal grain size number of No. 8.0 or above. The etching speed is improved and also the production of blurred periphery of pierced hole is reduced. Still, the flat mask prepared by this method is weak in brightness, which is left as an issue. The finest grain size number described in the patent is No. 10.0 which corresponds to 11 .mu.m of grain size. The grain size (B), (.mu.m), is calculated from the grain size number (A) by the following equation. EQU (A)=16.6439-6,6439.times.log{(B)/1.125}